The young censors in the Tianjin office – or “auditors” – work for Beijing ByteDance Technology Co, better known as Toutiao, a popular and fast-growing news feed app.

Surrounded by noodle restaurants and construction sites, the Wisdom Mountain Twin Towers, where the censors do their work, don’t exactly look Orwellian.

Workers scan into bright offices using iPads. There are team building sessions typical of start-ups the world over. And the dress code is casual.

“Our corporate culture is really good; every afternoon, for example, we get together for tea,” said one censor at the Toutiao office. A “horizontal” management structure means “ordinary employees can send messages about their issues straight to the CEO”.

The censor added: “Overall the firm is seen as a cool place to work.”

Toutiao’s Tianjin “auditing” centre is at the heart of a vast Chinese censorship effort that is growing fast as official scrutiny of online content intensifies.

According to figures released by the state media outlet Beijing News, China had roughly 2 million online content monitors in government departments and private companies in 2013. Academics estimate that number has since risen sharply.

The government has been tightening control over videos, chat platforms and social media ahead of a Communist Party congress in October at which Xi is expected to bolster his leadership.

Under Xi, the government has stepped up efforts to control discourse online as a growing array of web platforms give people new channels for self-expression.

“They control a lot already but are really cleaning up for the party congress,” said Tsui Lokman, a journalism professor at the Chinese University of Hong Kong.

He said the clampdown would last well beyond the congress and was having a widespread “chilling effect”.

Companies like Toutiao are responding, hiring armies of workers to police videos, blogs and news articles available to its 120 million users across China.

“We had about 30-40 employees two years ago; now we have nearly a thousand reviewing and auditing,” said the Toutiao censor, who, like other censors Reuters spoke to, asked not to be named due to the sensitivity of the topic.

A guard and receptionist at the building said the Tianjin office had expanded rapidly.

“Everyone here is doing auditing work,” the receptionist said. “One year ago there was one floor, now we have 10.”

Toutiao, which Reuters reported last month was raising at least US$2 billion in a new funding round that would value it at about US$20 billion, said it had been expanding its teams rapidly, including in content “auditing”.

“We have invested in developing sophisticated AI analytical tools and stringent content management processes to weed out low quality and fake content,” the company said in a statement, referring to artificial intelligence. The company declined to say how many censors it employed.

Reuters spoke to four Toutiao censors and four other staff, who described the company’s censorship work, which they said spiked during periods of activity by the country’s political leaders.

The censors rotate between day and night shifts; the peak time for censoring content is from 6pm to 9pm. Workers review videos, users’ posts and news, rooting out political criticism.

They also target topics ranging from violence and drug addiction to extramarital affairs and religious cults, all of which were blacklisted in lengthy guidelines issued in June.

“You can’t have anything that is too vulgar, too violent, too bloody, or anything that makes people feel disgusted,” said a second Toutiao censor based in Beijing, where the company has its headquarters. “There’s no set rules; more it’s the discretionary judgment of those on duty.”

Some topics are particularly sensitive – anything to do with Xi is automatically flagged by computers. Others are totally off limits.

The “6.4 tank event” – a reference to the date of the crackdown on student protests in Tiananmen Square in June 1989 – and “various nicknames for state leaders” are automatically blocked, the censors said.

Most of the censors said they were doing a public service.

“There is a lot of evil and pollution on the internet that people don’t see, and we are helping protect people,” a third Toutiao censor said.

But the efforts of the censors are often met with intense vitriol online by those whose posts are removed and others who decry the growing censorship in China.

“Looking for my friends’ posts, I find they’ve all been erased,” a Weibo user posted under the handle “Jue Nian”.

“I’m afraid in a few years that history will have been rewritten so many times there’ll be no space for opposing points of view.”

Beijing has tightened rules this year for internet companies to self-censor content on their platforms, and has fined web giants like Tencent Holdings, Baidu and Weibo for not doing enough to clean up content.

In-house censors work separately from government censors, who operate within state media and local propaganda units and liaise with private companies.

Weibo and Tencent, which operates the popular chat platform WeChat, did not respond to requests for comment.

Baidu declined to comment, but pointed to a statement from August saying it was committed to dealing with malicious information on its platforms.

Zhang Lijun, chairman of the online news and video portal V1 Group, said that between 20 and 30 per cent of his company’s labour costs went on content auditors – a necessary business expenditure.

“Without doubt you need to maintain close ties with the ruling party,” Zhang said. “Party building, setting up party units properly, these can ensure your news goes out smoothly and keeps your business operations safe.”

The Beijing-based censor said Toutiao used artificial intelligence systems to censor content, though these don’t always understand the tone of posts.

“We are training the AI. They are not as smart. Hopefully they will learn to handle all this eventually.”

For now, though, real humans are still in demand.

An advertisement Toutiao posted on Tianjin Foreign Studies University’s career page for students this month sought 100 fresh graduates to work in “content audit”, earning between 4,000-6,000 yuan ($611-$917) per month.

Successful candidates need to “love news and current affairs” but also be “politically savvy” and “understand the laws and regulations governing internet supervision”.

One advertisement, for a “forum auditor” posted on the recruitment site Lagou.com in September, said the person would be responsible for working with direction from China’s powerful internet regulator, the Cyberspace Administration of China.

The regulator did not respond to requests for comment.

Most postings are for young graduates, generally seen as more receptive to the job’s demands.

“People who have just graduated from college are clean like a white piece of paper, and will accept our corporate culture more easily,” said one Tianjin censor.